Thermal and photochemical initiation of propane cracking on structurally heterogeneous rare earth molybdates

Introduction

Energy and environmental challenges are critical for sustainable development. Propylene is a key feedstock for polymer production, yet current industrial processes are insufficient to meet increasing demand.

Methods

The phase composition and crystal structure of the samples were studied by X-ray diffraction (XRD). The textural characteristics and specific surface area were determined by low-temperature nitrogen adsorption. The catalytic properties of the samples were studied in the propane cracking reaction under conditions of a thermo- and photoinitiated process using ultraviolet irradiation.

Main results

Samples of Ln₂(MoO₄)₃ (Ln = La, Gd, Yb) were synthesized by co-precipitation. For the studied compounds, successive phase transitions are observed from the monoclinic phase C12/c1(15) to a mixture of the modulated I112/b(αβ0)00(15.1) and orthorhombic Pba2(32) phases, and then to a mixture of the tetragonal I41/a(88) and orthorhombic Pnca(60) phases. These structural transformations are of interest in terms of studying the catalytic properties of rare-earth molybdates in the catalytic cracking of propane. The use of photochemical initiation in the presence of these catalysts leads to a significant increase in the catalytic performance of the process, including propane conversion and light olefin selectivity, compared to thermally initiated cracking. For La₂(MoO₄)₃, propane conversion at 750 °C increases from 59% to 99% when UV irradiation is used. Furthermore, upon transition from a thermally initiated to a photoinitiated process, a decrease in ethylene selectivity and a simultaneous increase in propylene selectivity are observed for all studied lanthanide molybdates. The most pronounced increase in propylene selectivity is observed for Gd₂(MoO₄)₃: under photoinitiation, it reaches 68% at a propane conversion of 88%.

Conclusions

Phase transitions in Ln₂(MoO₄)₃ (La → Gd → Yb) and the increase in structural complexity correlate with changes in catalytic properties. More defect-rich and mixed phases promote the formation of active sites, which enhances the efficiency of the photocatalytic process.